1. Field of the Invention
The present invention relates generally to particle size analysis apparatus and more specifically to an improved mixing reservoir used in an automated recirculating particle size analysis system.
2. Discussion of the Related Art
Automated recirculating particle size analysis systems are used to prepare a slurry by suspending the particles to be measured in a liquid and to continually stir the slurry to provide a homogenous suspension. The slurry is then continuously recirculated through an analyzer during analysis. Typically such recirculator systems utilize a mixing chamber or reservoir in which a stirring impeller is disposed to thoroughly mix the particles. The slurry with its suspended particles is pumped from the mixing reservoir to the analyzer and then returned to the reservoir. The analysis apparatus conventionally includes a sample cell where an included analysis apparatus measures the particle distribution content of the slurry. One such automated recirculating particle size analysis system is disclosed by U.S. Pat. No. 5,439,288 to Jeffrey G. Hoffman et al., and which is now assigned to the same assignee as the present invention.
It is of vital importance in such systems that the distribution of the particles in the slurry in the sample cell be representative of the entire statistical population to ensure valid data collection for analysis.
In apparatuses that measure particle size in the micron particle size ranges of less than 100 microns, even the densest materials disperse uniformly throughout the fluid and provide a uniform slurry. However, particles in the size range between 100 to 1000+ microns have movement that tends to become more independent of the fluid. When the slurry is reintroduced to the mixing reservoir for recirculation, the large dense particles tend to fall straight to the exit port. Since large particles are under-represented in the mixing tank, they are over-represented in the sample cell of the measuring apparatus, therefore, developing a non-uniform distribution of particles within the recirculating apparatus. This non-uniform distribution does not accurately represent the statistical population of particles in the slurry.
Prior art methods for keeping large particles in suspension in the reservoir tank included the use of high-speed impellers for shifting and directing the particles away from the reservoir outlet and/or directing the return flow against a deflecting surface so as to deflect the returning particles into the larger volume of the reservoir. Such prior art devices have been found to have drawbacks in that the impeller typically introduces excess turbulence and bubbles within the reservoir due to the speed of the impeller and its propensity to cause cavitation in the fluid. Additionally, the larger delicate particles may be broken into smaller pieces when battered against hard surfaces during injection onto deflecting surfaces and, therefore, not correctly represent the particle size distribution found in the manufacturing process.